The 3rd Generation Partnership Project Radio Access Network Long Term Evolution (hereinafter, referred to as LTE) (3GPP-LTE) employs orthogonal frequency division multiple access (OFDMA) for the downlink communication scheme and single carrier frequency division multiple access (SC-FDMA) for the uplink communication scheme (see, NPLs 1, 2 and 3, for example). Moreover, periodic sounding reference signals (P-SRS) are used in the uplink of LTE as reference signals for estimating the uplink reception quality.
In order to transmit P-SRS from a terminal to a base station, an SRS transmission subframe which is common to all terminals (hereinafter, referred to as common SRS subframe) is configured. This common SRS subframe is defined by a combination of a predetermined periodicity and a subframe offset on a per-cell basis. In addition, the information on the common SRS subframe is broadcasted to terminals within the cell. For example, when the periodicity is equal to 10 subframes and the offset is 3, the third subframe in a frame (consisting of 10 subframes) is configured as a common SRS subframe. In a common SRS subframe, all the terminals within the cell stop transmission of data signals in the last SC-FDMA symbol of the subframe and use the period as the resources for transmission of reference signals.
Meanwhile, subframes for SRS transmissions are individually configured for terminals by a higher layer (i.e., RRC layer higher than the physical layer) (hereinafter, referred to as individual SRS subframe). Each terminal transmits P-SRS in the configured individual SRS subframe. In addition, parameters for SRS resources (hereinafter, may be referred to as “SRS resource parameters”) are configured and reported to each terminal. The parameters for the SRS resources include the bandwidth, bandwidth position (or SRS frequency domain starting position), cyclic shift and comb (corresponding to identification information on the subcarrier group) of the SRS, for example. The terminal transmits SRS using the resources specified by the reported parameters. Additionally, SRS frequency-hopping may be configured.
Meanwhile, the introduction of dynamic aperiodic SRS (hereinafter, referred to as A-SRS) into the uplink of LTE-Advanced, which is an advanced version of LTE (hereinafter, referred to as “LTE-A”) has been discussed. The transmission timing of A-SRS is controlled by trigger information (e.g., 1-bit information). This trigger information is transmitted to a terminal from a base station on a physical layer control channel (i.e., PDCCH) (e.g., see NPL 4). To put it more specifically, the terminal transmits A-SRS only upon request for A-SRS transmission made by the trigger information (i.e., A-SRS transmission request). In addition, there has been discussion on defining, as the transmission timing of A-SRS, the first common SRS subframe located after the fourth subframe from the subframe in which the trigger information has been transmitted. As described above, while terminals transmit P-SRS, periodically, terminals are allowed to transmit A-SRS in a concentrated manner within a short period only when uplink data transmissions occur in bursts, for example.
Moreover, LTE-A has control information formats for various types of data assignment reporting. The control information formats in the downlink include: DCI format 1A for allocation of resource blocks consecutive in number (Virtual RBs or Physical RBs); DCI format 1, which allows allocation of RBs not consecutive in number (hereinafter, referred to as “non-contiguous bandwidth allocation”); DCI formats 2 and 2A for assigning a spatial-multiplexing MIMO transmission; a downlink assignment control information format for assigning a beam-forming transmission (“beam-forming assignment downlink format”: DCI format 1B); and a downlink assignment control information format for assigning a multi-user MIMO transmission (“multi-user MIMO assignment downlink format”: DCI format 1D). Meanwhile, the uplink assignment formats include DCI format 0 for assigning a single antenna port transmission and DCI format 4 for assigning an uplink spatial-multiplexing MIMO transmission. DCI format 4 is used for only terminals in which uplink spatial-multiplexing MIMO transmission is configured. In addition, DCI format 0 and DCI format 1A are adjusted in size by padding so that each format consists of the same number of bits. DCI format 0 and DCI format 1A are also called DCI format 0/1A in some cases. DCI formats 1, 2, 2A, 1B and 1D are used in accordance with downlink transmission modes configured in each terminal (i.e., non-contiguous bandwidth allocation, spatial-multiplexing MIMO transmission, beam-forming transmission and multi-user MIMO transmission) and are formats to be configured in each terminal. Meanwhile, DCI format 0/1A can be used independently of the transmission modes and thus can be used for terminals in any transmission mode, i.e., DCI format 0/1A is a format commonly usable in all terminals. In addition, when DCI format 0/1A is used, single-antenna transmission or transmit diversity is used as the default transmission mode.
Terminals receive DCI format 0/1A and the DCI formats that are dependent on the downlink transmission modes. In addition, terminals in which uplink spatial-multiplexing MIMO transmission is configured receive DCI format 4 in addition to the DCI formats mentioned above.
In this respect, using DCI format 0 for reporting the trigger information for A-SRS has been discussed. DCI format 0 is a control information format used in reporting uplink data (PUSCH) assignment. The field for reporting the trigger for A-SRS is added to DCI format 0 in addition to RB reporting field, MCS reporting field, HARQ information reporting field, transmission power control command reporting field and terminal ID field. It should be noted that, A-SRS and P-SRS can be used together or singly. In addition, parameters for SRS resources (e.g., transmission bandwidth, cyclic shift and/or the like) are configured independently for A-SRS and P-SRS.